- Email: [email protected]
Grain populations in laser-crystallised silicon thin films on glass substrates
Thin Solid Films 383 Ž2001. 110᎐112
Grain populations in laser-crystallised silicon thin films on glass substrates b , J.-H. Werner b , M. Nerding a,U , S. Christiansen a , J. Krinke a , R. Dassow b , J.R. Kohler ¨ H.-P. Strunk a a
Uni¨ ersitat Institut fur ¨ Erlangen-Nurnberg, ¨ ¨ Werkstoffwissenschaften, Lehrstuhl fur ¨ Mikrocharakterisierung, Cauerstr. 6, D-91058 Erlangen, Germany b Uni¨ ersitat ¨ Stuttgart, Institut fur ¨ Physikalische Elektronik, Pfaffenwaldring 47, D-70569 Stuttgart, Germany
Abstract We investigate the polycrystalline microstructure, i.e. grain size and orientation distribution, that forms during laser crystallisation of amorphous silicon on glass substrates by a frequency doubled Nd:YVO4-laser operating at a wavelength of 532 nm. Transmission electron microscopy reveals that the grains have an average width from 0.25 to 3 m and a length of several 10 m. Electron back-scattering diffraction indicates that the grain orientation of the poly-Si films is textured. Type and extent of texturing depend in a complex way on the thickness of the crystallised amorphous silicon layer and on whether or not a buffer layer is present. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Transmission electron microscopy; Electron back-scattering diffraction; Laser crystallisation; Texture; Grain orientation distribution
1. Introduction Polycrystalline silicon Žpoly-Si. fabricated at low temperatures is of great interest for large area electronics on temperature sensitive, cheap substrates. Possible devices are thin film transistors ŽTFTs. in active matrix liquid displays w1x and thin film solar cells w2x on glass. Over the past few years a variety of techniques w3᎐6x were applied to improve the performance of the films essentially to control the defect population, density and location. Especially for thin film solar cells, where laser crystallised films are used as seeding layers for a subsequent epitaxial, low-temperature thickening process but also for TFTs, the surface normal needs to be controlled in many cases. The defect density produced by the most promising thickening processes Že.g. ion-assisted deposition, electron-cyclotron resonance chemi-
U
Corresponding author. Tel.: q49-9131-8528618; fax: q49-91318528602. E-mail address: [email protected] ŽM. Nerding..
cal-vapour deposition. shows a strong dependence on the crystallographic orientation of the seeding layer w7x with the 1004 surface normal direction having the lowest defect density. We analyse in this paper how the grain size and the texture are influenced by the process parameters and thereby give a first indication on how to control them. 2. Experimental The amorphous silicon Ža-Si. layers, deposited by sputtering on Corning 1737F glass substrates are crystallised by the so-called sequential lateral solidification process ŽSLS. w8x. This method is based on lateral epitaxy where the already formed grains act as seeds for the solidification process in the adjacent melt. An important aspect of this technique is that elongated, large grains, result only when the a-Si layer is completely molten when laser irradiated. This defines a minimum laser pulse energy Ždepending on film thickness. required for growth of large grained material.
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 6 2 3 - 0
M. Nerding et al. r Thin Solid Films 383 (2001) 110᎐112
Agglomeration limits the maximum possible pulse energy. Therefore, there is always a specific range of laser pulse energies connected with a certain film thickness. Details concerning the experimental set-up of the Nd:YVO4 laser-crystallisation process used are described elsewhere w9x. We investigate a-Si film samples with different thicknesses Ž50 nm, 75 nm, 150 nm, 300 nm. with and without a silicon-nitride ŽSiN. buffer layer. We study the dependence of grain size and texture on layer thicknessrpulse energy. Since laser crystallisation is a locally acting high temperature process and is carried out on ordinary borosilicate glass, diffusion of impurities from the glass into the silicon film has to be anticipated. Therefore, we also investigated samples with a silicon nitride buffer layer that acts as a diffusion barrier. Grain sizes are evaluated by transmission electron microscopy ŽTEM. techniques. The TEM investigations are carried out in a conventional Philips CM20 microscope operated at 200 kV. We use a commercial Oxford electron back-scattering diffraction ŽEBSD. setup in a JEOL 6400 scanning electron microscope and determine the three-dimensional grain orientations evaluating so-called pseudo-Kikuchi patterns. For each sample a matrix of several thousand measurements is obtained which represents the texture with good statistics. 3. Results 3.1. Grain size The grain size is given by the grain length Žessentially determined by the length of the scanned area. and the grain width d. The grain width correlates, as shown in Fig. 1, with the layer thickness. An increase of the pulse energy at constant layer thickness ŽFig. 1. results in an increase of the grain width. Optimised process conditions yield large grained material with grains ) 100 m long and 0.25᎐2.5 m wide.
Fig. 1. Correlation between average grain width and layer thicknessrpulse energy at constant layer thickness.
111
Fig. 2. Grain orientations of laser crystallised poly-Si films on glass at various layer thicknesses represented in the standard triangle as determined by EBSD.
3.2. Texturing We summarise our texture determinations as depending on film thickness Žand laser pulse energy range connected with it. for films without ŽFig. 2. and for films with ŽFig. 3. SiN buffer layer. For the films crystallised on glass we can conclude the following: the different textures are always found at characteristic film thicknesses. For the 50-nm thick layer the surface normals cluster close to ²233: Žfirst row of standard triangles in Fig. 2., for the 75-nm thick layers close to ²110:, and the thickest layer Ž300 nm. shows surface normal orientation preference between the ²113: and the ²013: direction. The second and third rows in Fig. 2 show two orthogonal in-plane directions, the scanning direction of the laser and the
Fig. 3. Typical grain orientation of laser crystallised poly-Si films on SiN plot in the standard triangle as determined by EBSD. No dependence on layer thickness observed.
112
M. Nerding et al. r Thin Solid Films 383 (2001) 110᎐112
rolling direction perpendicular to it, indicating that not only preferred surface normals form but a three-dimensional texture develops. The 150-nm-thick films are special: their texture depends on the pulse energy used for crystallisation. Pulse energies at the maximum of the pulse energy range connected with 150 nm film thickness lead to a texture very similar to 300 nm thick films, pulse energies at the minimum of this range lead to a texture similar to 75-nm-thick films. The texture of films crystallised on a SiN diffusion barrier is very pronounced but shows a completely different behaviour than without SiN layer. Here the preferred grain orientation is practically independent of the film thickness Žand pulse energy within the applicable range.. Fig. 3 shows the evaluated texture. The laser scan direction coincides preferably with the crystallographic ²110: direction, the rolling direction corresponds to the ²112: direction and the surface normal coincides with the ²111: direction. 4. Discussion Two main results of our investigations will be briefly discussed, first the increase of the grain width with layer thicknessrpulse energy and second the observed textures. The increase of the grain width with increasing pulse energy at constant layer thickness and with increasing film thickness Žsee Fig. 1. could be interpreted by the difference in the quenching rate. A smaller quenching rate connected with thicker films or higher energy impact keeps the melt longer at a low supercooling during the growth process. Referring to papers from w10x the larger the supercooling the BeckerrDoring ¨ larger the nucleation density and the smaller the maximum grain size that can be achieved. Applying this to our results a higher quenching rate connected with thinner layers or with lower pulse energy might lead to an increased contribution of nucleation from the melt, thus reducing the grain width, that can be achieved, an interpretation that coincides with our experiments. The appearance of textures is remarkable since it was so far not reported. However, we have to state that its interpretation is rather difficult in view of the limited results presently, especially in the case of glass substrates. The two interfacial scenarios, ‘on-glass’ and ‘on-SiN’ crystallisation show that texture formation depends on the interfacial conditions, one could think of the wetting behaviour of the melt. Wetting of SiN is stronger than that of glass w11x. It is known that wetting affects the nucleation energy in the sense that an increased wetting decreases the nucleation barrier. For the scanning direction a preference of the ²112: or ²110: directions could be expected as these are the fast growth directions w12x and grains oriented in that way laterally overgrow other grains. These directions have,
for reasons not yet understood, not been observed for the ‘on-glass’-system, while it is nicely shown for the ‘on-SiN’-system. The texture results of the 150-nm-thick films crystallised on glass indicate that the texture on glass rather depends on the laser pulse energy than on the film thickness. 5. Conclusion We crystallise with sequential lateral solidification amorphous silicon thin films with thicknesses between 50 nm and 300 nm using a frequency doubled Nd:YVO4 laser system. We present essentially two important findings: 1. The grain width correlates with the film thickness. 2. The grains show texturing, depending in a complex way on the pulse energy and on the interfacial conditions as given by the substrate. It is clear that there are more detailed investigations necessary until rules for texture engineering can be formulated. Acknowledgements We acknowledge access to the EBSD equipment in the Institute for General Materials Properties ŽProf. H. Mughrabi. at the University of Erlangen. References w1x T. Sameshima, M. Hara, S. Usui, Jpn. J. Appl. Phys, Part 1 28 Ž1989. 1789. w2x R.B. Bergmann, J. Kohler, R. Dassow, C. Zaczek, J.H. Werner, ¨ Phys. Stat. Sol. Ža. 166 Ž1993. 587. w3x J.S. Im, M.A. Crowder, R.S. Sposili, J.P. Leonard, H.J. Kim, J.H. Yoon, V.V. Gupta, H. Jin Song, H.S. Cho, Phys. Stat. Sol. Ža. 166 Ž1998. 603. w4x J.R. Kohler, R. Dassow, R.B. Bergmann, J. Krinke, H.P. Strunk, ¨ J.H. Werner, Thin Solid Films. 337 Ž1999. 129. w5x G. Aichmayr, D. Toet, M. Mulato, P.V. Santos, A. Spangenberg, S. Christiansen, M. Albrecht, H.P. Strunk, J. Appl. Phys. 85 Ž1999. 4010. w6x G. Andra, ¨ J. Bergmann, F. Falk, E. Ose, H. Stafast, Phys. Stat. Sol. Ža. 166 Ž1998. 629. w7x J. Platen, B. Selle, S. Christiansen, M. Nerding, M. Schmidbauer, W. Fuhs, MRS Spring Meeting San Francisco 2000, to be published; L. Oberbeck, T.A. Wagner R.B. Bergmann, MRS Spring Meeting San Francisco 2000 Žin press.. w8x R.S. Sposili, J.S. Im, Appl. Phys. Lett. 70 Ž1997. 767. w9x R. Dassow, J.R. Kohler, M. Nerding, H.P. Strunk, Y. Helen, K. ¨ Mourgues, O. Bonnaud, T. Mohammed-Brahim, J. Werner, Mat. Res. Soc. Symp. Proc. 621 Ž2000., to be published. w10x R. Becker, W. Doring, Ann. Physik. 24 Ž1935. 719. ¨ w11x C.K. Chen, L. Pfeiffer, K.W. West, M.W. Geis, S. Darak, G. Achaibar, R.W. Mountain, B-Y. Tsaur, Mat. Res. Soc. Symp. Proc. 53 Ž1986. 53. w12x J.L. Batstone, Phil. Mag. A 67 Ž1993. 51.
Grain populations in laser-crystallised silicon thin films on glass substrates b , J.-H. Werner b , M. Nerding a,U , S. Christiansen a , J. Krinke a , R. Dassow b , J.R. Kohler ¨ H.-P. Strunk a a
Uni¨ ersitat Institut fur ¨ Erlangen-Nurnberg, ¨ ¨ Werkstoffwissenschaften, Lehrstuhl fur ¨ Mikrocharakterisierung, Cauerstr. 6, D-91058 Erlangen, Germany b Uni¨ ersitat ¨ Stuttgart, Institut fur ¨ Physikalische Elektronik, Pfaffenwaldring 47, D-70569 Stuttgart, Germany
Abstract We investigate the polycrystalline microstructure, i.e. grain size and orientation distribution, that forms during laser crystallisation of amorphous silicon on glass substrates by a frequency doubled Nd:YVO4-laser operating at a wavelength of 532 nm. Transmission electron microscopy reveals that the grains have an average width from 0.25 to 3 m and a length of several 10 m. Electron back-scattering diffraction indicates that the grain orientation of the poly-Si films is textured. Type and extent of texturing depend in a complex way on the thickness of the crystallised amorphous silicon layer and on whether or not a buffer layer is present. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Transmission electron microscopy; Electron back-scattering diffraction; Laser crystallisation; Texture; Grain orientation distribution
1. Introduction Polycrystalline silicon Žpoly-Si. fabricated at low temperatures is of great interest for large area electronics on temperature sensitive, cheap substrates. Possible devices are thin film transistors ŽTFTs. in active matrix liquid displays w1x and thin film solar cells w2x on glass. Over the past few years a variety of techniques w3᎐6x were applied to improve the performance of the films essentially to control the defect population, density and location. Especially for thin film solar cells, where laser crystallised films are used as seeding layers for a subsequent epitaxial, low-temperature thickening process but also for TFTs, the surface normal needs to be controlled in many cases. The defect density produced by the most promising thickening processes Že.g. ion-assisted deposition, electron-cyclotron resonance chemi-
U
Corresponding author. Tel.: q49-9131-8528618; fax: q49-91318528602. E-mail address: [email protected] ŽM. Nerding..
cal-vapour deposition. shows a strong dependence on the crystallographic orientation of the seeding layer w7x with the 1004 surface normal direction having the lowest defect density. We analyse in this paper how the grain size and the texture are influenced by the process parameters and thereby give a first indication on how to control them. 2. Experimental The amorphous silicon Ža-Si. layers, deposited by sputtering on Corning 1737F glass substrates are crystallised by the so-called sequential lateral solidification process ŽSLS. w8x. This method is based on lateral epitaxy where the already formed grains act as seeds for the solidification process in the adjacent melt. An important aspect of this technique is that elongated, large grains, result only when the a-Si layer is completely molten when laser irradiated. This defines a minimum laser pulse energy Ždepending on film thickness. required for growth of large grained material.
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 6 2 3 - 0
M. Nerding et al. r Thin Solid Films 383 (2001) 110᎐112
Agglomeration limits the maximum possible pulse energy. Therefore, there is always a specific range of laser pulse energies connected with a certain film thickness. Details concerning the experimental set-up of the Nd:YVO4 laser-crystallisation process used are described elsewhere w9x. We investigate a-Si film samples with different thicknesses Ž50 nm, 75 nm, 150 nm, 300 nm. with and without a silicon-nitride ŽSiN. buffer layer. We study the dependence of grain size and texture on layer thicknessrpulse energy. Since laser crystallisation is a locally acting high temperature process and is carried out on ordinary borosilicate glass, diffusion of impurities from the glass into the silicon film has to be anticipated. Therefore, we also investigated samples with a silicon nitride buffer layer that acts as a diffusion barrier. Grain sizes are evaluated by transmission electron microscopy ŽTEM. techniques. The TEM investigations are carried out in a conventional Philips CM20 microscope operated at 200 kV. We use a commercial Oxford electron back-scattering diffraction ŽEBSD. setup in a JEOL 6400 scanning electron microscope and determine the three-dimensional grain orientations evaluating so-called pseudo-Kikuchi patterns. For each sample a matrix of several thousand measurements is obtained which represents the texture with good statistics. 3. Results 3.1. Grain size The grain size is given by the grain length Žessentially determined by the length of the scanned area. and the grain width d. The grain width correlates, as shown in Fig. 1, with the layer thickness. An increase of the pulse energy at constant layer thickness ŽFig. 1. results in an increase of the grain width. Optimised process conditions yield large grained material with grains ) 100 m long and 0.25᎐2.5 m wide.
Fig. 1. Correlation between average grain width and layer thicknessrpulse energy at constant layer thickness.
111
Fig. 2. Grain orientations of laser crystallised poly-Si films on glass at various layer thicknesses represented in the standard triangle as determined by EBSD.
3.2. Texturing We summarise our texture determinations as depending on film thickness Žand laser pulse energy range connected with it. for films without ŽFig. 2. and for films with ŽFig. 3. SiN buffer layer. For the films crystallised on glass we can conclude the following: the different textures are always found at characteristic film thicknesses. For the 50-nm thick layer the surface normals cluster close to ²233: Žfirst row of standard triangles in Fig. 2., for the 75-nm thick layers close to ²110:, and the thickest layer Ž300 nm. shows surface normal orientation preference between the ²113: and the ²013: direction. The second and third rows in Fig. 2 show two orthogonal in-plane directions, the scanning direction of the laser and the
Fig. 3. Typical grain orientation of laser crystallised poly-Si films on SiN plot in the standard triangle as determined by EBSD. No dependence on layer thickness observed.
112
M. Nerding et al. r Thin Solid Films 383 (2001) 110᎐112
rolling direction perpendicular to it, indicating that not only preferred surface normals form but a three-dimensional texture develops. The 150-nm-thick films are special: their texture depends on the pulse energy used for crystallisation. Pulse energies at the maximum of the pulse energy range connected with 150 nm film thickness lead to a texture very similar to 300 nm thick films, pulse energies at the minimum of this range lead to a texture similar to 75-nm-thick films. The texture of films crystallised on a SiN diffusion barrier is very pronounced but shows a completely different behaviour than without SiN layer. Here the preferred grain orientation is practically independent of the film thickness Žand pulse energy within the applicable range.. Fig. 3 shows the evaluated texture. The laser scan direction coincides preferably with the crystallographic ²110: direction, the rolling direction corresponds to the ²112: direction and the surface normal coincides with the ²111: direction. 4. Discussion Two main results of our investigations will be briefly discussed, first the increase of the grain width with layer thicknessrpulse energy and second the observed textures. The increase of the grain width with increasing pulse energy at constant layer thickness and with increasing film thickness Žsee Fig. 1. could be interpreted by the difference in the quenching rate. A smaller quenching rate connected with thicker films or higher energy impact keeps the melt longer at a low supercooling during the growth process. Referring to papers from w10x the larger the supercooling the BeckerrDoring ¨ larger the nucleation density and the smaller the maximum grain size that can be achieved. Applying this to our results a higher quenching rate connected with thinner layers or with lower pulse energy might lead to an increased contribution of nucleation from the melt, thus reducing the grain width, that can be achieved, an interpretation that coincides with our experiments. The appearance of textures is remarkable since it was so far not reported. However, we have to state that its interpretation is rather difficult in view of the limited results presently, especially in the case of glass substrates. The two interfacial scenarios, ‘on-glass’ and ‘on-SiN’ crystallisation show that texture formation depends on the interfacial conditions, one could think of the wetting behaviour of the melt. Wetting of SiN is stronger than that of glass w11x. It is known that wetting affects the nucleation energy in the sense that an increased wetting decreases the nucleation barrier. For the scanning direction a preference of the ²112: or ²110: directions could be expected as these are the fast growth directions w12x and grains oriented in that way laterally overgrow other grains. These directions have,
for reasons not yet understood, not been observed for the ‘on-glass’-system, while it is nicely shown for the ‘on-SiN’-system. The texture results of the 150-nm-thick films crystallised on glass indicate that the texture on glass rather depends on the laser pulse energy than on the film thickness. 5. Conclusion We crystallise with sequential lateral solidification amorphous silicon thin films with thicknesses between 50 nm and 300 nm using a frequency doubled Nd:YVO4 laser system. We present essentially two important findings: 1. The grain width correlates with the film thickness. 2. The grains show texturing, depending in a complex way on the pulse energy and on the interfacial conditions as given by the substrate. It is clear that there are more detailed investigations necessary until rules for texture engineering can be formulated. Acknowledgements We acknowledge access to the EBSD equipment in the Institute for General Materials Properties ŽProf. H. Mughrabi. at the University of Erlangen. References w1x T. Sameshima, M. Hara, S. Usui, Jpn. J. Appl. Phys, Part 1 28 Ž1989. 1789. w2x R.B. Bergmann, J. Kohler, R. Dassow, C. Zaczek, J.H. Werner, ¨ Phys. Stat. Sol. Ža. 166 Ž1993. 587. w3x J.S. Im, M.A. Crowder, R.S. Sposili, J.P. Leonard, H.J. Kim, J.H. Yoon, V.V. Gupta, H. Jin Song, H.S. Cho, Phys. Stat. Sol. Ža. 166 Ž1998. 603. w4x J.R. Kohler, R. Dassow, R.B. Bergmann, J. Krinke, H.P. Strunk, ¨ J.H. Werner, Thin Solid Films. 337 Ž1999. 129. w5x G. Aichmayr, D. Toet, M. Mulato, P.V. Santos, A. Spangenberg, S. Christiansen, M. Albrecht, H.P. Strunk, J. Appl. Phys. 85 Ž1999. 4010. w6x G. Andra, ¨ J. Bergmann, F. Falk, E. Ose, H. Stafast, Phys. Stat. Sol. Ža. 166 Ž1998. 629. w7x J. Platen, B. Selle, S. Christiansen, M. Nerding, M. Schmidbauer, W. Fuhs, MRS Spring Meeting San Francisco 2000, to be published; L. Oberbeck, T.A. Wagner R.B. Bergmann, MRS Spring Meeting San Francisco 2000 Žin press.. w8x R.S. Sposili, J.S. Im, Appl. Phys. Lett. 70 Ž1997. 767. w9x R. Dassow, J.R. Kohler, M. Nerding, H.P. Strunk, Y. Helen, K. ¨ Mourgues, O. Bonnaud, T. Mohammed-Brahim, J. Werner, Mat. Res. Soc. Symp. Proc. 621 Ž2000., to be published. w10x R. Becker, W. Doring, Ann. Physik. 24 Ž1935. 719. ¨ w11x C.K. Chen, L. Pfeiffer, K.W. West, M.W. Geis, S. Darak, G. Achaibar, R.W. Mountain, B-Y. Tsaur, Mat. Res. Soc. Symp. Proc. 53 Ž1986. 53. w12x J.L. Batstone, Phil. Mag. A 67 Ž1993. 51.